The reduction of low-frequency sound in pipes and piping systems is a problem of great practical importance. One approach to addressing this problem in air-filled pipes, which has received considerable attention, is the use of active noise control, or "antisound." An acoustic signal is generated by one or more sound sources placed in the system that destructively interferes with the unwanted noise field. State-of-the art microprocessors can be used as digital, adaptive filters to synthesize the appropriate cancellation signal or signals by sampling the sound field in the duct. Experiments conducted to date in air-filled pipes and ducts have shown the utility of active control for reducing noise consisting of pure tones, bandwidth-limited white noise, and transient pulses.
The acoustic response of liquid-filled pipes differs from that of air-filled pipes in several significant ways. For example, that in an air-filled pipe, the pipe wall typically acts as a rigid or nearly rigid structural element, whereas a liquid-filled pipe, the finite loop stiffness of the pipe exerts a major influence on the propagation velocity of an acoustic disturbance in the pipe. For this reason, results applicable to air-filled pipes are not directly translatable to liquid-filled ones.